1. Field of the Invention
The present invention relates to an image forming apparatus and an image density control method for controlling the image density at fixed density over a long period of time.
2. Description of the Related Art
In the recent electrophotography field, the movement of realization of high image quality is remarkable regardless of a printer, digital copying machine, analog copying machine, color copying machine, and monochromatic copying machine.
Among them, realization of high image quality of a full-color digital printer and copying machine and a monochromatic digital printer and copying machine is a key point of product development.
Image forming can be broadly divided into two parts such as image forming by an image forming unit side image (hereinafter called printer gamma) and image forming by the image processing art.
Important matters for realizing high image quality are improvement of stability regarding the image forming unit side and how to find image processing parameters according to various documents regarding the image process.
When the image forming capacity on the image forming unit side is unstable even if image processing parameters are satisfactory, images obtained finally are varied widely and high image quality cannot be obtained.
Therefore, to make an image forming unit image, that is, the printer gamma stable, an image quality maintenance device may be incorporated.
The image quality maintenance device is almost incorporated in full color copying machines and full color printers and used in a part of monochromatic copying machines and monochromatic printers whenever necessary.
The image quality maintenance device is referred to as a device for maintaining a fixed printer gamma within the range from initial (movement start) to life (end of forming of a fixed number of images) or even if the environmental conditions such as temperature and humidity are changed.
In principle, a toner adhesion amount reading sensor is used and toner is adhered onto a photosensitive drum so as to form a patch image in a case (for example, during warming up) other than the actual image forming (copying or printing) operation. The sensor reads the toner adhesion amount for forming the patch image on the drum and controls so as to obtain a target toner adhesion amount. Or, the patch image on the drum is transferred onto a transfer belt and the sensor reads the toner adhesion amount on the transfer belt and controls so as to obtain a target toner adhesion amount.
In order to obtain the target toner adhesion amount, as disclosed in Japanese Patent Publication 64-5291, at least one of the peak value of the development bias AC voltage, frequency, and waveform center value is changed. In order to obtain the target toner adhesion amount, the process conditions such as the charged voltage, development speed, laser (exposure) conditions, and transfer conditions are generally controlled.
The image quality maintenance device is broadly divided into two parts such as a one-point controller and a two-point controller. They are shown in FIG. 4. The one-point controller is a controller for controlling only by one half tone point and the two-point controller is a controller for controlling by two points of a low-density part and a high-density part.
The advantages and disadvantages of the image quality maintenance devices of one-point control and two-point control are shown in FIG. 7.
As to the fixing property of the printer gamma, that is, whether the printer gamma is fixed as a result of matching, needless to say, the two-point control for matching at two locations is advantageous. The one-point controller matches at one half tone point, so that the fixing property in the neighborhood of the half tone is realized, though the matching may be shifted at the low-density part or high-density part. For example, as shown in FIG. 8, compared with the printer gamma at initial time and the printer gamma at life time in the case of the one-point control, the printer gamma at life time at the low-density part is darker (the image density is higher) than the printer gamma at initial time.
When the printer gamma is changed at life time like this, in a case of a copying machine, a dark image like insufficient exposure is formed and a problem arises that the toner consumption is increased. In a case of a printer, characters may be made thick.
At the half tone part, they coincide with each other because it is an image quality maintenance matching point. At the high-density part, in this case, there is little change observed between the initial time and the life time.
As mentioned above, that the printer gamma shape (characteristic) at initial time does not coincide with that at life time is caused by deterioration of the photosensitive drum, differences in the laser conditions between the initial time and the life time deterioration of the developer, and others.
The difference in the gamma shape between the initial time and the life time is beyond comparison with the difference caused by the machine body difference (individual difference).
The example shown in FIG. 8 is an example indicating a gamma change that the low-density part at life time is darker than that at initial time. However, depending on the machine body use conditions represented by the characteristics of the photoconductor and developer and laser conditions, gamma changes different from those shown in FIG. 8 may be indicated.
As mentioned above, with respect to the gamma fixing property, the one-point control has a lower fixing property than that of the two-point control and a problem arises that a density difference is generated at life time.
However, the two-point controller requires a toner adhesion amount sensor for accurately reading the wide density range of low density and high density. As a result, it is expensive and a problem arises that the convergence time is prolonged. In the two-point control, since the convergence time is long, a considerable time may be required before starting printing or copying and the toner consumption may be increased, so that it is desirable to make the convergence time as shorter as possible.
As explained above, the one-point controller is outstanding in the price and convergence time, while the two-point controller is advantageous in the function for making the required printer gamma constant.
An object of the present invention is to provide an image forming apparatus and an image density control method which are improved in disadvantages of a one-point controller and can be applied to such an apparatus represented by a digital full-color copying machine that a gamma fixing property on a high level is required.
According to the present invention, an image forming apparatus is provided and the image forming apparatus comprises a charging means for charging a photoconductor, an exposure means for exposing the photoconductor and forming an electrostatic latent image on the photoconductor, a development means for visualizing the electrostatic latent image with toner which forms a predetermined reference image using the toner, a detection means for detecting the toner density of the reference image, a means for setting a target density value of the reference image formed on the photoconductor according to the number of images to be formed, and a control means for changing at least one of the charging potential to be given to the photoconductor from the charging means, the exposure amount at the time of exposure from the exposure means to the photoconductor, and the development bias voltage given to the development means on the basis of the comparison result of the value detected by the detection means with the set target density value.
Furthermore, according to the present invention, an image density control method in an image forming apparatus including a charging means for charging a photoconductor, an exposure means for exposing the photoconductor and forming an electrostatic latent image, and a development means for visualizing the electrostatic latent image with toner is provided and the image density control method comprises a step of forming a reference image by the development means, a step of detecting the toner density of the reference image, a step of setting a target density value of the reference image formed on the photoconductor according to the number of images to be formed, and a step of controlling so as to change at least one of the charging potential to be given to the photoconductor from the charging means, the exposure amount at the time of exposure from the exposure means to the photoconductor, and the development bias voltage given to the development means on the basis of the comparison result of the value detected at the detection step with the target density value set at the setting step.